The invention relates generally to flow sensors, and more particularly, to variable orifice flow sensors.
Orifice flow sensors are used to measure the flow rates of fluids, which include liquids and gases. A typical orifice flow sensor comprises a fixed orifice through which a fluid is made to flow. A pressure difference is established between the fluid that is present upstream from the orifice and the fluid that is flowing through the orifice. This pressure difference can be used to measure the flow rate of the fluid. For this purpose, a pressure transducer measures the pressure difference that is established across the orifice, and is calibrated such that the flow rate of the fluid is calculated from this pressure difference.
Variable orifice flow sensors provide sufficient pressure difference for measurement purposes across a broad range of flow rates. This is achieved by introducing a flapper into the fluid flow passage. The flapper bends or flexes in the direction of the fluid flow and hence creates a variable orifice. The measurement of flow rates in a variable orifice flow sensor is similar to the measurement of flow rates in fixed orifice flow sensors. That is, a pressure transducer measures the pressure difference across the variable orifice and calculates the flow rate of the fluid from the pressure difference.
U.S. Pat. Nos. 4,989,456; 5,033,312; 5,038,621; and 6,722,211 show variable orifice flow sensors.
Variable orifice gas flow sensors are commonly used for measuring flow rates in medical applications, such as breathing apparatuses that deliver desired quantities of breathing gases to a patient. When used to measure breathing gases or recirculating breathing gases, the gases flowing through the sensor may contain moisture.
In the case where a gas flowing through a variable orifice flow sensor includes moisture, liquid droplets may accumulate next to the flapper. These liquid droplets restrict the bending of the flapper in the direction of flow of the gas. Therefore, the size of the variable orifice is reduced, and hence, the pressure difference across the variable orifice becomes altered. This leads to inaccurate measurements of the flow rate of the gas.